Chinese is notoriously difficult for native Western language speakers to learn—but now neuroscience might explain why.

A group of researchers has discovered that Chinese children adopted into French-speaking families at an age of 12 months still register Chinese tones later in life, despite having no conscious understanding of the language itself. The study, published in the Proceedings of the National Academy of Sciences (PNAS), contradicts previous conclusions regarding the staying power of first languages. Until now, studies have indicated that the childhood language of adoptees is probably wiped from the brain as children learn—and become fully immersed in—a new language.

But the recent PNAS report focuses on Chinese, a tonal language in which a single word could have multiple meanings depending on its patterns of rising or falling pitch, or tone.

Adoptees might not remember how to speak their first language, but the linguistic sounds from when they were very younger may still be embedded in the brain.

The experiment involved 49 girls aged between nine and 17 in the Montreal area. The girls fell into three groups: monolingual French speakers with no exposure to Chinese, girls bilingual in French and Chinese, and the Chinese adoptees. All groups were asked to listen to “pseudo words” that used the tones prevalent in Chinese languages. MRI scans revealed that the adoptees showed the same brain activity as native speakers, despite no longer being able to understand and speak anything in the language.

Fred Genesse, professor emeritus at the psychology department at McGill University and co-author of the report, highlighted the significance of the MRI results. He said: “In most people when you process language your left hemisphere is involved. When the monolinguals are listening to these pseudo words, they’re not processing them as language. For them it just sounds like a jumble of sounds. When you look at the two other groups, the areas of the brain they are activating are in the left hemisphere, so they are treating these pseudo words as linguistic units, as words.”

The finding is, at least for now, limited to one specific language transition: Chinese to French. And since no modern Chinese dialect is atonal, experts can’t necessarily assume that the same stark similarity in brain activity between natives and adoptees would apply for other language transitions that don’t rely on pitch variations to endow meaning.

On the other hand, the study’s participants were adopted at an average age of 12.8 months, around the time that speech acquisition begins. The babies, then, probably would have been able to isolate the sounds of their local tongue—suggesting that if the seed for language has sprouted by the time of adoption, then some acoustic elements of that first language (Chinese or not) may be recognized by the brain as language—and not just noise—later on in life.